As development of light-weighted construction and high performance for automotive vehicles has progressed, high stress design has been required for valve springs in automotive engines, suspension springs, clutch springs, brake springs, and the like.
For instance, a low sag resistance of a spring may increase the sag amount of the spring while a high load stress is exerted to the spring. As a result, the rotating speed of the engine may not be raised as expected in the design, thereby leading to poor responsiveness. Therefore, there is a demand for springs having superior sag resistance.
There is known that use of a high-strength spring material is effective in improving sag resistance of springs. Also, it is conceived that use of the high-strength spring material is effective in improving fatigue properties of the springs from the viewpoint of fatigue limit. For instance, there is known a technique of improving fatigue strength and sag resistance of springs by regulating the chemical composition of the spring material, and by increasing the tensile strength of the spring material after quenching and tempering, namely, after an oil tempering process. Also, there is known a technique of improving sag resistance of springs by adding a large quantity of an alloy element such as silicon (Si) to the spring material (see Japanese Patent No. 2898472, and Japanese Unexamined Patent Publication No. 2000-169937).
Despite these efforts, springs may encounter breakage trouble in an attempt of improving fatigue properties and sag resistance by increasing the tensile strength of the spring material. Further, in an attempt of improving sag resistance by adding a large quantity of an alloy element, resultant springs may have excessively high sensitivity to surface flaws and internal defects. As a result, it is highly likely that the springs suffer from breakage trouble resulting from the defective parts in assembling or in use.
As mentioned above, it is not easy to improve sag resistance and fatigue properties of springs without sacrificing workability (cold workability) of the spring material.
In view of the above, it is an object of the present invention to provide a steel wire for high-strength spring, and high-strength springs having superior sag resistance and fatigue properties without sacrificing workability (cold workability) of the steel wire.